EU and world leaders are locked in behind-the-scene talks about how to deal with the fresh terror threat to the West.

Top-level discussions are taking place between intelligence agencies - including gardai with responsibility for state security - who are liaising with CIA, Interpol and other international organisations.

The terrorist attack in Paris was raised as the United Nations met for its General Assembly on the Year Ahead.

The high state of alert comes in the wake of two days of bloodshed in Paris in which 10 journalists and three police officers were gunned down.

France remained on a state of high alert last night as the net closed on the perpetrators.

World leaders expressed outrage over the killing of 10 journalists and two policemen in the attack on the satirical magazine 'Charlie Hebdo', with France's allies, Muslim-majority countries and the biggest developing nations all condemning the murders.

But a Government minister here has warned European leaders have to deal with the security issues of "violent fundamentalism".

Three masked gunmen stormed the newspaper's Paris offices on Wednesday. The publication has long mocked religion and has run cartoons of the Prophet Mohammed.

EU leaders such as German Chancellor Angela Merkel and British Prime Minister David Cameron contacted French President Francois Hollande to offer support after the deadly gun attack.

As the search for the 'Charlie Hebdo' assailants continued, Government ministers here signalled Ireland would participate on a Europe-wide basis in a security response. Defence Minister Simon Coveney said his department was in the process of devising plans aimed at defending the country from new threats of terrorism. Mr Coveney said the range of different threats facing countries is constantly changing.

He said the country must remain vigilant following the "shocking" terrorist attack in Paris, adding that "violent fundamentalism" remains a big issue in Europe.

"This is very much an issue that Europe has to face and deal with, in terms of defence and security capacity, but also ensuring that communities with real diversity can live together without extremism trying to force and intimidate a change that would be totally unwelcome in terms of shutting down freedom of speech," he said.

"We are vigilant all of the time in terms of assessing threats to Ireland - whether that's cyber crime, terrorism, inclement weather - whatever the threat is, the responsibility of the Department of Defence is of course towards conventional defence, but also to assess new threats that maybe we wouldn't have considered as serious threats in the past and put an infrastructure in place to deal with them in the future."

Four leading French imams and the Vatican issued a joint declaration denouncing the Paris newspaper massacre and warning that the world is a dangerous place without freedom of expression, but urged the media to be respectful of religions.

The Vatican's office for inter-religious dialogue said the four Muslim spiritual leaders, who were visiting the Vatican this week, joined Pope Francis in condemning the attack and urging all believers to show friendship and solidarity to the victims.

The declaration, issued in French, stressed that dialogue among faiths was the only way to eliminate prejudice.

"In these circumstances, we should recall that the world is in danger without freedom of expression," it said.

Chancellor Merkel said the "heinous act" is not only an attack on the life of French citizens and the internal security of France. "It also represents an attack on freedom of opinion and freedom of the press, a core element of our liberal democratic culture, that is completely unjustifiable," she said.

Mr Cameron said Britain "stands united with the French people" in its opposition to all forms of terrorism.

Russian President Vladimir Putin said his country was ready to continue to co-operate in combating terrorism.